Filter base



FIG. 1 is a top, front right perspective view of a fourth embodiment of a filter base shown according to the present design;

FIG. 2 is a bottom, back right perspective view thereof;

FIG. 3 is a front end view thereof;

FIG. 4 is a rear end view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a left side elevational view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines show portions of a filter base which form no part of the claimed design. 

CLAIM The ornamental design for a filter base, as shown and described. 